The present invention generally relates to television cameras having an optical lowpass filter, and more particularly to a television camera having an optical lowpass filter which prevents the generation of color beat, and prevents degradation of the resolution.
In a television camera which uses a single image pickup tube, a color separating optical system including a color stripe filter is used. However, in this optical system, beat is introduced between the high-frequency component of the luminance signal and the chrominance signal. Accordingly, in order to reduce the high-frequency component of the luminance signal so as to reduce the above beat, an optical low-pass filter (generally having an attenuation pole) is generally employed.
As a conventional optical lowpass filter of the above type, there are those which use a crystal plate and those which use a polyhedron prism. Further, there are optical lowpass filters which use bi-prisms previously proposed by the present inventors.
However, in the above type of a conventional optical lowpass filter used in a television camera, the optical lowpass filter is constructed so as to provide an optical lowpass filter effect throughout the entire surface of an effective light path of a beam which reaches an image pickup surface of the image pickup tube through an iris. Accordingly, since the optical lowpass filter is originally used to prevent the generation of the color beat, the generation of the color beat is naturally prevented by the use of the optical lowpass filter. However, because the high frequency range part of the beam is attenuated, the resolution is degraded. For example, there was a disadvantage in that fine stripes could not be picked up clearly.